1. Field of the Invention
The present invention relates to a tone evaluation apparatus and tone evaluation method of evaluating the tone in a color conversion table for converting image data on the first color space into data on the second color space.
2. Description of the Related Art
One of the important factors that determine image quality is the tone of an image. For example, when a color image is on an L*a*b* space, its tone has a three-dimensional distribution. For this reason, in order to evaluate the performance of a color image forming apparatus, it is very important to evaluate the tone of an image in a three-dimensional space.
As a method of evaluating the tone of an image in a three-dimensional space, there is proposed a method of performing so-called objective evaluation for an image output by a color image forming apparatus or the like (see, e.g., Japanese Patent Laid-Open No. 2005-174133). According to this image evaluation method, a gradation image is read, color information such as lightness, chroma, and hue are acquired, and then correction considering the human visual characteristics is applied to the color information.
Subsequently, the difference of the corrected color information between the neighboring pixels of the gradation image is calculated, and the difference is compared with a threshold to calculate a tone jump position and a tone jump amount on the gradation image. Furthermore, the accumulated value of the acquired tone jump amounts for the entire gradation image is calculated as a tone evaluation value.
With the above-described processing, a tone evaluation value for at least one color information can be calculated, or the weighted sum of tone evaluation values acquired for two or more pieces of color information can be calculated as an overall evaluation value.
In the tone evaluation method described in Japanese Patent Laid-Open No. 2005-174133, however, only the color space reproduced on the output gradation image can be evaluated.
Generally, a color image forming apparatus converts input image data on the first color space (e.g., RGB) into data on the second color space (e.g., L*a*b*) as a uniform color space so as to perform various processes. In addition, the image data which underwent processing on the second color space is converted into data on the third color space (e.g., CMYK) for output. In so-called color conversion processing for conversion between color spaces, a color conversion table is used. Since the content of the color conversion table influences the quality of a formed image, it is necessary to evaluate tone reproduction by the color conversion table.
When the tone evaluation method described in Japanese Patent Laid-Open No. 2005-174133 is to be applied to a color conversion table, it is necessary to output a gradation image which covers the whole color space reproducible using the color conversion table. Furthermore, in order to detect a tone jump position, it is necessary to output a gradation image which surely includes the tone jump. However, it is difficult to generate such a gradation image. In addition, since a tone jump position calculated by the tone evaluation method described in Japanese Patent Laid-Open No. 2005-174133 is presented as a position on the gradation image, it is difficult to pinpoint that portion of the conversion table, which has a problem.
The present invention has been made to solve the above-described problems, and has as its object to provide a tone evaluation apparatus and tone evaluation method comprising the following functions. That is, for a color conversion table used when performing color conversion from the first color space to the second color space, a tone in the entire area of an available gamut is easily and appropriately evaluated. In addition, a tone jump position on the color conversion table is effectively specified.